I've been using HelloChinese and one of the quizzes it gave (without much explanation) was something like ni men ai mao ”你们 爱 猫“ to mean "you love cats". However, reading it I thought it was "you all love cats" (like the southern US slang Y'all). In google Translate it seems to mean the same thing with or without 们
What exactly does men add to the meaning of this phrase and where should it be used?
Generally,Speaking will involve two roles: speaker and audience.
When audience is only one person, you can use pronoun '你' to represent the listener. However, when the audience is more than one person and you want to refer to them all, you can use pronoun '你们' to represent everyone of audiences.
Imagine a scene： while you play basketball with five partners, in addition, four people in the opposite half to play basketball. Now, you want everyone to play with together and tell them your idea.
You might say:" Can 我们 play with 你们？"
It is a bit weird to say 你们爱猫。 Is that an order, a statement or a question?
你们爱猫吗？ 皇帝命令：你们都一定要爱猫也！
If you are speaking at the Annual General Meeting of the Furry Friends Association you might begin thus:
欢迎朋友们！晚上好！
我知道，你们都爱猫。
The 们 in 你们 or 我们 just marks the plural, for all use 都。
y'all and yins both seem to be African American and fairly recent (not more than 200 years old) in origin. The slaves brought their own African languages with them, y'all probably comes from a transliteration of an African language.
http://www.cantonese.sheik.co.uk/dictionary/characters/5/
們:
an adjunct to a pronoun or noun to indicate plurality
Pronoun:
你 (you) is a singular pronoun in Chinese, Adding 們 after it make it a plural pronoun- 你們(you)
我 (I / me) is a singular pronoun in Chinese. Adding 們 after it, makes it a plural pronoun- 我們 (we / us)
他 (he / him) is a singular pronoun in Chinese. Adding 們 after it, makes it a plural pronoun- 他們 (they / them)
Noun:
The adjunct 們 is mostly used with human pronouns (e.g. - 你們, 我們, 他們). Although it can be used with a noun, it is restricted to nouns for human only.
For example:
戰士 warrior / 戰士(們) warrior(s)
學生 student / 學生(們) student(s)
工人 worker / 工人(們) worker(s)
人 person ; people / 人們 (people) * 人 can be either singular or plural
Using 們 with nouns is much less common and it can often be omitted, because plurality can often be indicated by use of plural classifiers (e,g. - a group of; a class of) or plural adjective (e.g. - those; these).
們 is always used on people or other situations when you want to express personification.
If we write 一群學生 (a group of students) or 那些學生 (those students) we no longer need 們 to indicate plurality. We know it is not a single student.
Adding 們 and write 一群學生們 or 那些學生們 would be redundant.
The plural of you is you. But in Chinese, the plural of you is 你们。
Where are you getting 'all' from?
你们爱猫吗？
Do you like cats?
咱们（都）很喜欢猫！
We （all) really like cats!
( we 咱们, includes the speaker and the people addressed, no longer extant in English. Old English had wit for this. Now just use we.)
If you have 50 cats at home, I might say:
看起来你们很喜欢猫！
Looks like you (guys) really like cats!
你 is singular you. 你们 is plural you. 们 (which can only be used for pronouns or nouns referring to people) makes it plural.
"You all" may be a more unambiguously plural translation, but plain old "you" is still more common even when referring to multiple people. So that may be why HelloChinese and Google Translate say "you" instead of "you all". Google Translate also translates Spanish vosotros and ustedes, which are plural, as "you", although it currently translates 你们 as "you guys".
ni men. Just confirmation that ni is a pronoun and can be used withmen. No explanation on whatmenwould add in meaning though